Continued effort will be made to develop monoclonal antibodies specific for various populations of human mononuclear cells. In particular, those reactive to subpopulations of B cells but not reactive to surface Ig and Ia antigens will be sought. They will be used to ascertain the heterogeneity of the leukemic cells in chronic lymphocytic leukemia. Suitable mutant human T and B cell lines with deficiency in hypoxanthine phosphoribosyl transferase will be developed for fusion with T and B cells. Hybrids will be characterized as to the surface antigen expression and the products they make.